Magnetic resonance imaging (MRI) has become a leading tool for imaging fine details of anatomy and physiology as well as functional imaging.
MRI offers certain known advantages as a non-invasive imaging technology. For example, MRI can potentially provide exceptionally high anatomic resolution approaching single-cell levels (voxel of 20-40 μm3). Recent innovations in instrument design and contrast agent development indicate that the above level of resolution can be achieved non-invasively in vivo. Moreover, MRI can be used at tissue depths where optical reporting methods can sometimes be complicated by light scattering and absorption by the tissue, e.g., tissue depths greater than about 250 μm. One of the future directions of in vivo MRI research includes mapping of specific molecules (e.g., receptors) and detecting patterns of their expression.
In both clinical and research settings, MRI techniques can benefit from the use of biocompatible contrast agents (CAs), which enhance the image contrast by shortening of proton relaxation times (T1 and T2) of water molecules. This shortening of T1 and T2 produces subtle local MR signal changes, which can be detected, giving rise to enhanced signal-to-noise ratios and, in many cases, providing reasonably exact spatial locations if the MR signal changes are mapped versus non-influenced water molecules in a target tissue volume. Accordingly, it is desirable for in vivo imaging applications that CAs exhibit relatively high atomic relaxivities, r1p, which is defined as the shortening of water proton relaxation rates in presence of CAs, normalized per concentration of a paramagnetic element. In some instances, the r1p of some CAs can be enhanced by covalent or noncovalent association of the CAs with a preformed macromolecule, e.g., a protein, a polypeptide, a dendrimer, or a graft-copolymer.
It has previously been demonstrated that reducing substrates can be made paramagnetic. For example, hydroxytyramine and serotonin can be acylated with derivatives of monosubstituted DOTA(Gd) (gadolinium salt of 1,4,7,10-tetraazacyclododecane-1,4,7-tris(acetic acid)) and successfully used as an electron donor in horseradish peroxidase and myeloperoxidase-catalyzed reaction of hydrogen peroxide reduction. Peroxidase reduction generates radicals that polymerize rapidly. For example, formation of polymers has been demonstrated in the case of tyrosine. Polymerization of these low molecular weight paramagnetic molecules results in an increase of proton relaxivity, i.e., the ability of gadolinium to shorten relaxation times of water protons. As a result, the enzyme-mediated conversion of the substrate into polymerized products was detected using magnetic resonance imaging (MRI).